Water purification units for use in laboratories have been devised that remove levels of contaminants to very low levels. They typically contain a variety of technologies that remove particles, bacteria, ionic species and organic molecules. Water is recirculated within the unit to maintain the highest purity. When a user has required water he has typically opened a valve to dispense the amount of water required. A user will typically open the valve fully at first and partially close the valve as the amount of water dispensed approaches that required. This allows the user to make sure the amount of water does not exceed fill lines typically found on laboratory equipment.
U.S. Pat. No. 5,925,240 discloses an improved system of controlling the water output of the unit by varying the pump speed and/or the outlet valve's cross section in conjunction with a timer to give a desired amount of water. Both the pump and the valve can introduce an inaccuracy into the amount dispensed. Pump speed is typically controlled by reducing the voltage applied to the pump and correlation between this voltage and water output can vary depending upon inlet water pressure, temperature, back pressure and any gas hold up. Valve throughput can also be affected by pressure and temperature.
WO 01/27798 A discloses the use of a flow sensor to determine the amount of water dispensed by a deionization system and charge accordingly. It also includes a safety feature wherein the flow is halted if readings from the flow and pressure sensors indicate a missing/failed nozzle.
EP1134190A discloses a sensor fitted to a laboratory water purification unit which measures how much water has passed through the unit. A controller changes the pump speed between two speeds and opens a dispense valve to dispense an amount of water input into the units controller by the user.